Natural materials have some restrictions on their electromagnetic properties. It is difficult to find a natural material which has the necessary properties or material parameters required for certain applications. This problem usually can be solved by using metamaterials.
Metamaterials (hereinafter referred to as MTMs) are defined as artificial materials which achieve extraordinary electromagnetic properties that are not possible in natural material. Although composite materials have been investigated for many years, MTMs really form a new field of science and engineering. If one considers the first demonstration to be from 2000-2001 by Pendry et. al. as the commencement of MTMs, this field is less then 10 years in 2008. See, Composite Medium with Simultaneously Negative Permeability and Permittivity; Smith, D. R., Padilla, W. J., Vier, D. C., Nemat-Nasser, S. C., Schultz, S.; 2000, Physical Review Letters 84 (18), pp. 4184-418.
The index of refraction of MTMs is considered to be negative when the effective permittivity and permeability are simultaneously negative. As a consequence of their double negative parameters, plane waves propagating in such metamaterials are characterized by antiparallel phase and group velocities and reversal of wave vector that follow a left-hand rule, thus giving rise to the name left-handed (meta)materials (LHM). This type of material has been named by Veselago as left-handed material (LHM) V. G. Veselago, The electrodynamics of substances with simultaneously negative values of ε and μ, Sov. Phys. Usp., 10, pp. 509-514, (1968).
Experiment and simulation have confirmed that the LHMs do indeed exhibit negative refraction. R. A. Shelby, D. R. Smith, S. Schultz, Experimental verification of a negative index of refraction, Science, 292 (5514), pp. 77-79, (2001). However, the experiment by Shelby was conducted in the microwave region and the sample investigated was a bulk three-dimensional prism slab.
Although THz-TDS has been proposed as a technique for measuring the index of refraction of MTMs, see, J. Han, Probing negative refractive index of metamaterials by terahertz time domain spectroscopy, Opt. Exp., 16 (2), pp. 1356, (2008), a negative index of refraction in the terahertz region has not yet been observed. Materials exhibiting a magnetic response are extremely rare in THz and optical frequencies because magnetic phenomena tend to occur at far lower frequencies. Certain ferromagnetic and antiferromagnetic materials such as InGaMnAs/InP and GdMnO3 exhibit a very weak magnetic response over a frequency range of several hundred gigahertz which limit the scope of possible THz devices. Nearly all of the MTMs that exist in the terahertz region are single, two-dimensional layers fabricated on a rigid substrate due to the limitation of the conventional MEMs technique. This is mainly due to the inability in fabricating the MTMs into a bulk three-dimensional slab. Thus there is a need for alternative, advanced and simplified MTMs that can be easily fabricated and characterized, and methods for making such MTMs.